A primary objective of photo microlithography is to create images of the finest detail possible. High resolution images having extremely narrow line and space widths permit manufacture of complex integrated circuits on chips of small size.
To form a pattern on a semiconductor chip, light is transmitted through a planar mask and projected onto a planar wafer by a projection lens that generates a reduced image of patterns on the mask onto the wafer. Light in the projected image causes a chemical reaction in photo-sensitive material (photoresist) coated on the wafer, such that a relief image of the mask is created on the wafer. Illumination of the mask plane and of the wafer plane must be uniform, avoiding both small, local variations, and broad macroscopic variations across either plane.
In lithographic processes for the fabrication of semiconductor devices, a mask pattern is transferred to a substrate by the projection method for the sake of increased yield of products and better resolution. In the projection method, as the consequence of a trade-off between the resolution of the transferred pattern and the size of an area exposed in each exposure operation, it is prevailing to employ a step-and-repeat or scan-and-repeat technique with an apparatus called a stepper or a scanner which accomplishes exposure of the entire area of the substrate by repeating the sequence of making an exposure on a limited area of the substrate, then moving the substrate a predetermined distance and making another exposure.
Recent trends of further miniaturization of the patterns on an integrated circuit have forced development of optical lithography using light in the deep UV region, i.e. having a wavelength in the range of 190 to 330 nm. In such optical lithography, a super Hg lamp or an Xe-Hg lamp is often used as the light source. Since, however, the Hg lamp or Xe-Hg lamp has substantially no directionality and provides poor luminance, use of such lamps as the light source results in prolonged exposure time and, thus, in a decreased throughput
Development of advanced lasers which can provide laser beams in shorter wavelength regions, such as solid state lasers and excimer lasers make it possible to introduce laser energy of high luminance in the deep UV region into an exposure apparatus for the manufacture of semiconductor circuit devices. In addition, laser diodes in the ultraviolet have already been demonstrated in laboratory settings. GaN lasers lasing at approximately 360 nm have been demonstrated.
One of the main problems with conventional steppers (or scanners) is the lack of good control of the light source illuminating the mask, which due to the high numerical aperture (NA) and high resolution of the current exposure tools has become an extremely important issue. In conventional tools, mask illumination is provided (or their equivalent) by light emerging from a two-dimensional array of lens lets which is in turn illuminated by a Hg arc lamp or an eximer laser through an assembly of optics in between. Hence, it is relatively difficult to configure the light source into various shapes for improved resolution and depth of focus with regards to exposing different patterns since it is difficult to turn on/off light from each lenslet separately